Road Bike Crank Test

As a lot of you tuning in already know, we’ve been working on our in-house road crank tests for the last 7 years, with our updates posted alongside all the previously tested cranks. One thing we’ve come to realize is that while some cranks have obvious changes over the years, others have quietly evolved behind the scenes. Because of this and a desire to build a new testing fixture, particularly one that would handle BB30 cranks as well, we decided it was time to retire the old tests and start again with a clean slate. Please note that because of the new fixture and testing loads, results from this and future tests are not comparable to results from previous tests. Being that only 2 of the first 12 cranks tested in the last round carry over from previous tests we felt this was a good time to make the change.

Luckily, Jason Krantz is back as our resident data-cruncher (Jason is an experienced mechanical engineer with a strong background in both finite element analysis and composite materials.) We’re always happy to have him work with us on our tests and place a high value on his contributions.

Disclaimer: A lot of typing and numbers have gone into this article and we apologize in advance for any typos, but would warn that the possibility of mistakes is present.

About the testing method: Each arm was preloaded with 50 lbs of weight, then all calipers and load cells were zeroed out. Another 150 lbs was added and the difference was measured in mm. Each arm was tested three times and an average of those measurements is the result. A lower number represents a stiffer crank. These will be labeled as Deflection-D(Drive side deflection) and Deflection-ND(non-drive side deflection) . This method uses a total load of 200 pounds, in comparison to the old test which used a total load of 250 pounds. Further more in previous tests a large solid rod was used to simulate the pedal and load was applied 60mm from crank arm. With the new fixture, we’ve switched to using actual steel pedal spindles removed from a set of production pedals. The loading has also been moved slightly inward toward the crank arm.

Stiffness/Weight: Is determined by: (1/average deflection)/weight)x10000

Notes about stiffness: According to conventional wisdom, the more pedaling stiffness the better. Stiffness implies efficiency, along with the notion of a stiffer bike inspiring confident and responsive handling. As desirable as those traits are, and even after a crank’s stiffness is proven empirically, the outcome still seems subjective—we are simply going by how many people feel more efficient on a bike with a stiff drivetrain. But it’s far from clear whether that tactile, from-my-quads-directly-to-the-road sensation is actually faster. This is the question: is a stiffer drivetrain actually more efficient, or does it just feel that way? Looking at the numbers, we can see that average deflections range from roughly 0.20 inches to 0.30 inches. From this, we can generalize and say that the most flexible crank is about 50% more flexible than the stiffest crank. It’s easy to imagine that the stiffer cranks feel better, or have better “power transfer,” which is a particularly vague and ill-defined concept.

So how can we move beyond “feel” and attempt to quantify whether a stiffer crank is better? The answer is, strain energy. Strain energy is simply the energy stored by an object as it is loaded. Quantifying how much energy stored by a given spring under a particular load is a basic problem that works perfectly well for understanding whether a stiff bicycle crank is better than a slightly less stiff crank. That is, you can think of a bicycle crank as a very stiff, oddly-shaped spring. The equations for calculating stored energy in a beam under bending are fairly simple. The following figure is taken from a Creative Commons-licensed engineering textbook by Piaras Kelly at the University of Auckland:

In the equation above, U is strain energy, M is applied moment (torque), L is beam length, E is the Young’s modulus (stiffness) of the beam material, and I is the moment of inertia of the beam cross-section. A crank is somewhat beam-like, but it’s not really a beam. And we’re not applying a pure moment but rather a force at a distance that creates a moment. This bending example is somewhat close, but it’s still not a very good approximation of a crank on a bicycle. We can get a very good approximation of a crank on a bicycle by using finite element analysis (FEA). To find out how much strain energy a typical crank stores, we can solve an FEA model to find the strain energy of each of the constituent elements. We can then add up all of those strain energies to get the strain energy of the whole crank. This strain energy can then be converted into absorbed power by assuming a cadence; we used 100 RPM for this example. In this way, we can determine exactly how much power goes into crank flex, which can then tell us how much crank flex matters to total power output. We used ANSYS, a well-regarded FEA program, to model a generic aluminum left crankarm (172.5mm) and half an attached 24mm steel bottom bracket spindle.The remote force works out to 250 pounds of force (lbf), and it is applied 60mm to the outside of the center of the pedal threads. This simulates applying the force through a pedal.

Most of the action is happening on the inside of the crank; the interior elements report a higher strain energy value than the external ones do. But things get a lot more interesting when we dump the strain energy values for each of the elements to a spreadsheet. By summing the elements’ strain energy values, we can get the total strain energy for the entire crank and half BB spindle. The total strain energy of this crank under a 250-lb pedaling load is 4.604 Joules. As a unit, Joules don’t do much for most cyclists. We can convert them to a more useful unit by assuming a cadence of 100 RPM. At that cadence, this half-crank soaks up 7.67 Watts. The right-hand crank is usually stiffer than its left-hand counterpart, so it stores correspondingly less strain energy. So rather than doubling the left-hand figure, we’ll round down a bit to 14 Watts for the entire crank/BB axle system. 14 Watts might sound like a lot, and it is. But let’s keep in mind that this is a 250-lb force applied 1.67 times per second. A rider applying this force over 160 degrees of crank rotation produces an average power of 880 Watts, which few of us can sustain for long. And 14 Watts out of 880 is 1.6% To put this in perspective, if you were pedaling along at a steady 300 Watts, your crank would be absorbing 4.8 Watts of your effort. But those 4.8 Watts go into winding up your crank “spring,” which will spring back with nearly all the energy that was spent winding it up. Some of that spring-back energy probably helps turn the drivetrain while some of it may behave in a negative manner. However, there’s a fair amount of debate about how much energy is returned. The answer to the energy-return question involves kinematic analysis far outside the scope of this article. For now, we’ll assume that all of those 4.8 Watts spring back in a way that doesn’t help turn the drivetrain nor hinder it. As mentioned before, the most flexible crank in this review shows about 50% more deflection than the stiffest crank. Our FEA crank is quite flexible, and it absorbs 4.8 Watts of a 300-watt effort. Strain energy, roughly speaking, is inversely proportional to stiffness. We can use these relationships to calculate that at 300 Watts, our flexible crank absorbs 4.8 Watts, or 1.6% of total power output. Meanwhile, a 50% stiffer crank absorbs 3.2 Watts, or 1.07%, in strain energy (technically, strain power). That’s a difference of 1.6 Watts (or 4.7 watts at our tested 880 Watts). And remember, this assumes that no strain energy is returned to the drivetrain. That’s not to say that crank stiffness is irrelevant, there is a measurable difference. It also provides all the psychological and “feel” benefits described at the beginning of this section. A stiff crank also incrementally improves efficiency by keeping bearings aligned, keeping the pedals more directly beneath your feet, etc. Keep in mind that this 1.6% calculated strain energy absorption represents an upper bound of energy dissipation—that is, the maximum possible energy loss due to crank flexure. Real cranks are much stiffer than our modeled crank and therefore store less strain energy. Moreover, a great deal of that stored strain energy is likely returned to the drivetrain. The real-world losses to crank flexure are, in all likelihood, a fraction of that number.

Things get slightly more complicated for carbon cranks. Composite materials really do damp vibrations better than most metals, and cyclists treasure this property to ease their stinging hands as they hammer over Belgian cobbles and hop curbs on their way to the local coffee shop. But damping is just another word for energy absorption, and a carbon crank certainly absorbs and dissipates more energy than a metallic crank. Modeling that dissipation requires, among other things, intimate knowledge of the crank’s construction (“laminate schedule,” in the argot of composites engineering) But even if we can’t quantify carbon crank losses due to damping, are they worth worrying about? Almost certainly not. They are a tiny addition to the already-small losses in a metallic crank.

Notes about Crank Length: Over the years a lot of different arguments have been made about the benefits of longer/shorter cranks. None of which has really been thoroughly tested until Jim Martins study. Martin showed that length didn’t statistically matter when it came to power, once power was averaged around the entire pedal circle and not just in the forward position, it turns out that shorter cranks (down to 145mm) produced more average power than a longer crank. This conclusion however considers only average power and not other factors which definitely have a bearing on real world use. Damon Rinard followed up the Martin study with some of his own testing comparing the aerodynamic differences in crank length. In almost every case there was an aerodynamic improvement with the shorter crank and without a loss in power. So the power advantage and aerodynamic advantage, combined with shorter cranks generally allowing for a more aggressive or more comfortable position on the bike and less chance of repetitive motion injury we feel that shorter cranks are something most people should consider. We’re not saying they’re right for everyone, but if you’re on the fence as to which size is best for you, we suggest that you go for the shorter. If you’re interested in more on crank length we suggest reading the above articles as well as this article written by Frank Day for USA Cycling.

Notes about weight: Some cranks claimed weights include rings and bb in their complete weights while others do not. To have a level and fair comparison all cranks in this test will be tested and weighed with their stock bsa bottom brackets, and any crank that does not include rings will use our Praxis rings and KCNC alloy chainring bolts.

Notes on prices: Prices are MSRP for the U.S. and include rings and bottom bracket.

We do plan on adding more cranks to the review as they are released and as testing is completed. In choosing the cranks to start the new review with we listened to feedback from previous tests. We went with the most popular cranks as well as some of the lightest options. One suggestion that kept coming up in past tests was that people wanted to see how an older square taper crank did, so in this test we’ve included one.

Note that in comparison to square taper all the external-cup type cranks are worlds stiffer.

Now Onto the Results

** Fits only BB30/PF30 frames.

Graphs? Yeah, sure, why not?

Thoughts on Each Crankset

Final Thoughts

Now that you’ve gotten through the entire test, or skipped to the end, we suppose you’re looking for a summary. Well, we aren’t really going to offer much other than to say that when it comes to cycling there really isn’t a universal best. There is a best for a given set of circumstances. Each rider has to decide the aspects that are important for them and their needs, be it weight, aesthetics, price, size, BB compatibility etc… Hopefully we’ve provided enough information to help each rider determine which crank is best for them.